1. Field of the Invention
The invention relates to semiconductor processing chambers and, more particularly, the invention relates to gas distribution plates for a narrow gap chamber for deep trench etch.
2. Description of the Background Art
Integrated circuit (IC) wafer processing systems, particularly those which fabricate VLSI circuits on silicon wafers, can use many processes to form the circuit features in a die on a wafer. One of the more popular processes is magnetically enhanced reactive ion etching (MERIE) where a highly reactive plasma is used to react with the material on the wafer surface or an underlying substrate though a series of photoresist masks to produce the desired circuit features. A rotating magnetic field, produced by magnets mounted outside the chamber stirs the plasma. MERIE processes and reactors are described in detail in U.S. Pat. No. 5,215,619, issued Jun. 1, 1993 and U.S. Pat. No. 5,225,024, issued Jul. 6, 1993, both of which are incorporated herein by reference. A typical MERIE chamber has a pedestal for supporting a wafer. The pedestal typically includes a cathode and a mechanical or electrostatic chuck. Reactive gas enters the chamber through an aluminum gas distribution plate disposed above the pedestal. Typically, the gas distribution plate is attached to the underside of an aluminum lid that closes the top of the chamber. The gas distribution plate also includes a plastic blocker plate that occupies most of the space between an interior surface of the gas distribution plate and a bottom surface of the lid.
When MERIE is used to etch deep trenches in the surface of a semiconductor wafer, a narrow gap between the cathode and the gas distribution plate is often desirable. In this way the plasma is confined to a small volume within the narrow gap thereby increasing the plasma density without increasing the plasma power. The higher plasma density is desirable in a deep trench etch because it leads to a higher etch rate.
Prior art MERIE chambers have attempted to narrow this gap by mechanically changing the height of the pedestal. Such a height adjustable pedestal is expensive and time consuming to manufacture. As an alternative, the chamber lid may be designed such that the lid is indented. A MERIE chamber of the prior art is depicted in FIG. 1. The chamber 100 has a set of walls 102 defining an internal volume. A wafer 104 (shown in phantom) rests on a pedestal 106 situated inside the chamber 100. Lift pins 108 raise and lower the wafer relative to the pedestal 106. The wafer 104 is introduced into the chamber 100 by a robot arm 110 (also shown in phantom). Plasma confining magnetic fields are produced by magnets 138 mounted outside the chamber.
The chamber 100 is covered by a lid assembly 112. The lid assembly 112 includes an indented lid 114 that projects into the chamber 100. The lid 114 has a radially projecting flange 117. The lid 114 is supported on the chamber walls 102 by the flange 117 and secured thereto by bolts 116. The lid 114 is typically sealed by an O-ring (not shown). The lid 114 has a lower surface 115 that is substantially flat. A gas distribution plate 118 is attached to the lower surface 115 of the lid 114 by a plurality of long bolts 128 that fit through a plurality of clearance holes 129 in the lid 114 and thread into a plurality of tapped holes 130 in an upper surface 119 of the gas distribution plate 118. The indentation of the lid produces a narrow gap between a bottom surface of the gas distribution plate and the pedestal 106 that confines a plasma to a small volume within the chamber 102.
A plastic blocker plate 120 fits in a recess 121 in the upper surface 119 of the gas distribution plate 118. Reactive gas is fed into the chamber 100 through a gas feed line 122 which communicates with a passage 124 in the lid 114 and a matching hole 123 in the blocker plate 120 into the recess 121. Gas enters the chamber through a plurality of orifices 126 in the gas distribution plate 118 that communicate between the recess 121 and the interior of the chamber 102. A large diameter O-ring 132 is located radially outward of the clearance holes 129. A small diameter O-ring 134, located radially inward of the clearance holes 129. As shown in FIG. 1B, the O-rings seal the gas distribution plate 118 when the chamber is at room temperature (approximately 20.degree. C.). However, thermal stresses occur when the chamber is at its operating temperature of 70.degree. C. to 90.degree. C. These thermal stresses cause the lid 114 to bow downwardly at its center and upwardly at its rim as shown in FIG. 1C. As a result, a downward stress is applied to the blocker plate 120. The blocker plate 120, in turn, presses down on the gas distribution plate 118 exerting a stress that tends to cause first the inner O-ring 132 then the outer O-ring 134 to fail.
The lid 114 and the gas distribution plate 118 join at an interface 131 that terminates inside the chamber 100. A first vacuum leak path exists at the clearance holes 129, along the interface 131 past the large diameter O-ring 132. The interface 131 also communicates with the recess 121, therefore a second leak path exists through the clearance holes 129 along the interface 131 past the small diameter O-ring 134 into the recess 121 and thence through the orifices 126 into the chamber. As such the space in between the O-rings 132 and 134 is essentially at atmospheric pressure and, therefore, likely to leak into the chamber. Consequently, the chamber takes a long time to pump down. Residual moisture and gas adversely affect the result of a deep trench etch. If the chamber is not pumped down long enough to remove residual moisture and gas, contaminant particles (such as silicon oxide) can be formed on the wafer during processing rendering one or more dies on the wafer defective. Furthermore, each time the chamber is opened, both the lid 114 and the gas distribution plate 118 must be wet cleaned which delays wafer production.
Therefore, a need exists in the art for a lid assembly for a narrow gap MERIE reactor that remains sealed under operating conditions.